Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

W. F. Wedin


Field research was undertaken in Uruguay (1) to study legume proportion effects on N[subscript]2 fixation and transfer by white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and birdsfoot trefoil (Lotus corniculatus L.), each grown with tall fescue (Festuca arundinacea Schreb); (2) to estimate N fertilizer effects on N[subscript]2 fixation in first-year or established swards; and (3) to study legume and N fertilizer effects on soil N. Two small-plot experiments were planted in 1983 and 1984 with legume and grass seed rates to obtain four swards ranging from 30 to 90% legume (herbage yield) in each mixture. Fixed N was estimated by [superscript]15N isotope dilution using tall fescue pure stands as reference. Legume N derived from the atmosphere (%Ndfa) in first harvests was 58% for birdsfoot trefoil (BT), and 74% for white clover (WC) and red clover (RC). Values of %Ndfa were higher (90%) in winter and spring with no differences between species. Maximum 2-year herbage fixed-N was 500 kg ha[superscript]-1 and was exhibited by a RC pasture. Increasing percentage legume decreased %Ndfa linearly with similar slopes for the three species. As legume proportion increased, N[subscript]2 fixed per-area increased linearly for RC and BT but the increase for WC was curvilinear with a plateau at high WC proportions. Thus, strongly WC-dominated swards are not needed to optimize N[subscript]2 fixation. Transferred N increased with time, comprising up to 60% of grass-N. The annual amount transferred was similar for WC and RC but in summer, N transfer and soil nitrate were higher for the WC mixtures. Over time, transfer by BT became lower than for the clovers. A one-time N rate of 100 kg ha[superscript]-1 depressed %Ndfa values temporarily, only while the increased soil ammonium and nitrate remained high. Negative effects of N on annual fixed-N yield and legume stands were stronger for BT and in the seeding year. It is concluded that different patterns of growth in the legumes and the grass interact with mineral soil-N levels and are major factors influencing N[subscript]2 fixation.



Digital Repository @ Iowa State University,

Copyright Owner

Antonio Pedro Mallarino



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215 pages